<template>
  <div>
    <div class="menu">
      <div class="menu-left">
        <ul>
          <li class="menu-item" v-for="(menu,index) in menus" :key="index" :class="{current: index === currentIndex}" @click="clickList(index)" ref="menuList">
          <p class="text">{{ menu.name }}</p>
          </li>

        </ul>
      </div>
      <div class="menu-right" ref="itemList">
  <ul>
    <li class="cate" v-for="(menu, index1) in menus" :key="index1">
      <h4 class="cate-title">{{ menu.name }}</h4>
      <ul class="cate-item">
        <li v-for="(item, index2) in menu.sub" :key="index2">
          <a href="#" class="cate-item-wrapper">
            <router-link class="cate-item-wrapper" :to="{name: 'goods_list', params: {category_id: item.id}}">
            <div class="cate-item-img">
               <img :src="item.image" alt="">
                </div>
                <span>{{ item.name }}</span>
            </router-link>


          </a>
        </li>
      </ul>
    </li>
    <li class="cate-bottom"></li>
  </ul>
</div>

    </div>
  </div>
</template>

  <script>
  

import BScroll from 'better-scroll'
export default {
  data () {
    return {
      menus: [],
      rightLiTops: [],  // 右菜单每项的高度
      scrollY: 0        // 记住右菜单的滚动距离
   

    }
  },
  watch: {
  menus () {
    // $nextTick用来在下次DOM更新循环结束之后执行延迟回调
    this.$nextTick(() => {
      this._initBScroll()      // 初始化better-scroll
      this._initRightHeight() // 初始化右边菜单的高度
    })
  }
},
computed: {
  currentIndex () {
    // 当 scrollY 发生改变时，修改左菜单的激活项
    const {scrollY, rightLiTops} = this
    // 从右菜单中查找元素，返回元素索引
    return rightLiTops.findIndex((top, index) => {
      if (index === rightLiTops.length - 2) {
    return true
  }

      if (scrollY >= top && scrollY < rightLiTops[index + 1]) {
        this._initLeftScroll(index)
        return true
      }
    })
  }
},


  methods: {
    
  // 单击左菜单中的某一项后，将右菜单切换到对应项下面
    clickList (index) {
      this.scrollY = this.rightLiTops[index]
      this.rightBscroll.scrollTo(0, -this.scrollY)
},
    // 右菜单滚动时，左菜单联动
   

    // 初始化better-scroll
    _initBScroll() {
    this.leftBscroll = new BScroll('.menu-left', {
      click: true,
      mouseWheel: true
    })
    this.rightBscroll = new BScroll('.menu-right', {
      click: true,
      mouseWheel: true,
      probeType: 3  // 实时派发scroll事件
    })
    this.rightBscroll.on('scroll', (pos) => {
  this.scrollY = Math.abs(pos.y)
})

  },
  _initRightHeight () {
  let itemArray = []
  let top = 0
  itemArray.push(top)
  let allList = this.$refs.itemList.getElementsByClassName('cate')
  // 将 allList 转换为普通数组进行遍历，记录每个元素的高度
  Array.prototype.slice.call(allList).forEach(li => {
    top += li.clientHeight
    itemArray.push(top)
  })
  let bottom = this.$refs.itemList.getElementsByClassName('cate-bottom')[0]
  bottom.style.height = this.$refs.itemList.clientHeight / 1.2 + 'px'

  this.rightLiTops = itemArray
},
_initLeftScroll (index) {
  let menu = this.$refs.menuList
  let el = menu[index]
  // scrollToElement(el, time, offsetX, offsetY, easing)
  this.leftBscroll.scrollToElement(el, 300, 0, -100)
}



},

  created () {
    this.$indicator.open({
      text: '加载中'
    })
    this.$http.get('category').then(res => {
      this.$indicator.close()
      this.menus = res.data.data
    })
  }
}
</script>

<style lang="scss" scoped>
ul {
  margin: 0;
  padding: 0;
}
li {
  list-style: none;
}
.menu {
  display: flex;
  position: absolute;
  text-align: center;
  top: 40px;
  bottom: 50px;
  width: 100%;
  overflow: hidden;
  .menu-left {
    flex: 0 0 80px;
    width: 80px;
    background: #f3f5f7;
    .menu-item {
      height: 54px;
      width: 100%;
      .text {
        width: 100%;
        line-height: 54px;
        margin-bottom: 0;
      }
    }
    .current {
      width: 100%;
      background: #fff;
      .text {
        color: red;
      }
    }
  }
  .menu-right {
    flex: 1;
    background: #fff;
    .cate {
    height: 100%;
    .cate-title {
       margin: 0;
      text-align: left;
      font-size: 14px;
      color: #333;
      font-weight: bold;
      padding: 10px;
    }
    .cate-item {
      padding: 7px 10px 10px;
      display: flex;
      overflow: hidden;
      flex-flow: row wrap;
      li {
        width: 33.3%;
        .cate-item-wrapper {
          .cate-item-img {
            width: 100%;
            img {
              width: 70px;
              height: 70px;
            }
          }
          span {
            display: inline-block;
            font-size: 14px;
            color: #333;
          }
        }
      }
    }
  }

  }
}
</style>

